A breathing mask of this class has become known from German Patent No. DE-PS 962,313. An indicator, which indicates whether the filter insert attached to the mask still retains toxic gases to a sufficient extent, is arranged on the breathing mask in the visual range of the mask user. The indicator is applied to a support and is freely exposed to the gas present in the interior of the mask.
The prior-art indicator is intended preferably for detecting exhaustion of the filter, and displays a color reaction when toxic substances have penetrated into the interior of the mask. In a first embodiment, the support with the indicator is arranged directly on the inside of the mask window, and in a second embodiment, individual windows are provided in the area of the mask body, and the support with the indicator is arranged under these windows. Together with the indicator on the support, the windows form a detection device for the toxic substance.
One disadvantage of the prior-art detection device is that toxic substances may reach the indicator even before the breathing mask is put on. However, it is necessary for certain applications, e.g., during the testing of breathing masks, to activate the indicator only after the mask has been put on, in order for the color reaction not to take place under the effect of possible toxic components already present in the ambient atmosphere.
A dosimeter with an indicator layer exposed to the substance to be detected has become known from Canadian Patent CA-A 1,271,399. The indicator layer is covered with a strip that can be peeled off, which is to prevent the substance to be detected from reaching the indicator layer. The strip is peeled off before use, as a result of which the indicator layer will be exposed to the substance to be detected.
The prior-art dosimeter is used such that it is always directly accessible, i.e., the strip covering the indicator layer can be pulled off by the user immediately before putting into operation. Activation of such a dosimeter in a room not accessible to the user is not disclosed in this document.